Referring to FIG. 1, a computer is generally comprised of, among other elements, a motherboard (10), a central processing unit (CPU) (12), memory (14), and a plurality of circuit cards (16) for controlling components, performing functions, and the like. Most of these elements are inserted or otherwise electrically connected to the motherboard. Computer system components are generally connected via buses (18) or an electrically-conductive path traced along the motherboard. These buses are used for data transfer among the components. Further, power is delivered to the motherboard through a power connection (20). Then, depending on the component, power is supplied indirectly from the motherboard (10) or directly via a power connection on the component. In certain systems, the elements can be removed from or inserted into the computer while the system is running, i.e., the elements can be xe2x80x9chot-swapped.xe2x80x9d There exist standard specifications that allow the combination of components from different manufacturers. ISA (Industry Standard Architecture) is a bus specification that is based on the specification used in the IBM PC/XT and PC/AT. PCI (Peripheral Component Interconnect) is a local bus specification developed for 32-bit or 64-bit computer system interfacing. Most modem computers have both an ISA bus for slower devices and a PCI bus for devices that need better bus performance. Another specification, VME (VersaModule Eurocard bus), is a 32-bit bus widely used in industrial, commercial, and military applications. VME64 is an expanded version that provides 64-bit data transfer and addressing.
While it is generally cost effective to have most of the circuitry on a single large motherboard for desktop computers, such a configuration has certain drawbacks that are particularly important to industrial applications. Because the motherboard is usually thin and large enough to flex, breakage of small traces and solder joints on fine pitch surface mount devices may occur when plug-in boards are inserted. The occurrence of such breakage requires motherboard replacement, which in turn requires complete disassembly and reassembly of the computer system.
Particularly in industrial applications, such disassembly and reassembly, and the accompanying downtime, may be unacceptable. Also, given the rapid development of motherboard technology, finding an exact replacement for a motherboard can be difficult or impossible. Further, substitution of a non-exact replacement may cause software problems due to BIOS changes, changing device drivers, and different timing. Thus, standard specifications have been developed for systems and boards designed for use in industrial and telecommunications computing applications.
The PCI-ISA passive backplane standard defines backplane and connector standards for plug-in passive backplane CPU boards that bridge to both PCI and ISA buses. The PCI-ISA passive backplane standard moves all of the components normally located on the motherboard to a single plug-in card. The motherboard is replaced with a passive backplane that only has connectors soldered to it.
CompactPCI is a specification for PCI-based industrial computers that is electrically a superset of PCI with a different physical form factor. CompactPCI uses the Eurocard form factor popularized by the VME bus.
In the PCI specification, it was possible to select a single value for the pull-up resistor that would satisfy the requirement for both 3.3V and 5V backplanes. Therefore, it was possible to create Universal Signaling Environment capable cards. There is a mechanism defined by the PCI specification where the xe2x80x9csignaling environmentxe2x80x9d of the bus is defined by the value of the pins receiving the input/output (I/O) voltage, i.e., the VIO pins (either 3.3V or 5V). Thus, a universal card uses the I/O voltage VIO to define its own I/O voltage, rather than fixing it at 5V or 3.3V.
The CompactPCI bus architecture supports the 3.3V signaling environment, the 5V signaling environment, and hot swap. These features have the following corresponding requirements. The 3.3 V signaling environment requires 2.7 K Ohm (xcexa9) (+/xe2x88x925%) pull-up resistors. The 5V signaling environment requires 1.0 Kxcexa9 (+/xe2x88x925%) pull-up resistors. Hot Swap requires that all pins be biased at 1V (+/xe2x88x9220%) using a minimum 10Kxcexa9 pull-up resistor. Further, the Compact PCI specification has the additional requirements of a 10xcexa9 series termination resistor on every signal within 0.6xe2x80x2 of the connector pin, no more than 10 Pico-Farad (pf) capacitive load on any shared bus signal on a non-system slot board, and no more than 20 pf capacitive load on any shared bus signal on a system slot board.
There are two types of xe2x80x9cuniversalxe2x80x9d boards: Universal signaling environment and universal slot location. Universal signaling environment means that a board can operate in either a 3.3V or 5V bus backplane. With the original PCI specification, it was possible to select a value for the bus pull-up resistor that satisfied the specification for both the 3.3V and 5V signaling environments. With the new CompactPCI Specification, it is no longer possible to select a single resistor. Therefore, in order to be a universal signaling environment capable CompactPCI board, a board must provide both 2.7 Kxcexa9 (+/xe2x88x925%) and 1.0 Kxcexa9 (+/xe2x88x925%) pull-up resistors and provide a way to enable them correctly depending on the signaling environment.
Universal slot location describes a board that can function in either the system slot or non-system slot of a CompactPCI backplane. A system slot board is required to provide the common bus resources for the CompactPCI backplane, namely: bus pull-ups, bus clock, and the bus arbiter. A system slot board is allowed additional capacitive load per signal pin because of these additional features. In order to be CompactPCI Hot Swap Specification compliant, every signal pin must be biased to (1V +/xe2x88x9220%) through a minimum 10 Kxcexa9 resistor prior to insertion into a live or xe2x80x9chotxe2x80x9d backplane.
Those skilled in the art will appreciate that other requirements exist in the full CompactPCI, Hot Swap, and Passive Backplane PCI-ISA specifications which are available from PCI Industrial Computer Manufacturers Group of Wakefield, Mass. and are hereby incorporated in their entirety by reference.
In one aspect, a device for automatically varying resistance comprises a comparator for comparing a control voltage to a reference voltage; a switch operatively coupled to the comparator; and a first resistor and second resistor operatively coupled in a series connection between a pull-up voltage and a signal line. The switch is operatively coupled in a parallel connection with the first resistor and, based on the comparison between the control voltage and the reference voltage, the switch selectively bypasses the first resistor.
In one aspect, a method of automatically varying resistance comprises comparing a control voltage and a reference voltage; pulling-up a signal line to a pull-up voltage through a first resistor and a second resistor operatively connected in series if the comparison has a first outcome; and pulling up the signal line to the pull-up voltage through only the second resistor if the comparison has a second outcome.
In one aspect, an apparatus for automatically varying resistance comprises means for comparing a control voltage and a reference voltage; means for pulling-up a signal line to a pull-up voltage through a first resistor and a second resistor operatively connected in series if the comparison has a first outcome; and means for pulling up the signal line to the pull-up voltage through only the second resistor if the comparison has a second outcome.
In one aspect, a computer system for automatically varying resistance comprises a voltage supply for supplying a reference voltage and an operating voltage; a signal line requiring a pull-up resistance of a differing value depending on the operating voltage; a comparator for comparing the reference voltage and the operating voltage; a pass gate controlled by an output of the comparator; a first resistor operatively coupled between the signal line and a first side of a parallel connection of the pass gate and a second resistor; and a second side of the parallel connection of the pass gate and the second resistor operatively coupled to the operating voltage.
In one aspect, a custom integrated circuit (IC) for automatically varying resistance comprises a comparator for comparing a input/output voltage to a reference voltage operatively coupled to a first comparator terminal and second comparator terminal of the custom IC; an NMOS transistor having the output of the comparator operatively coupled to a gate of the NMOS transistor, and a source and a drain of the NMOS transistor operatively coupled across a 1.7 Kxcexa9 resistor; and the 1.7 Kxcexa9 resistor and a 1.0 Kxcexa9 resistor operatively coupled in series between a first pull-up terminal of the custom IC and a second pull-up terminal of the custom IC. The source of the NMOS transistor and a first side of the 1.7 Kxcexa9 resistor are operatively coupled to the first pull-up terminal of the custom IC, the drain of the NMOS transistor and a second side of the 1.7 Kxcexa9 resistor are operatively coupled to a first side of the 1.0 Kxcexa9 resistor, and a second side of the 1.0 Kxcexa9 resistor is operatively coupled to the second pull-up terminal of the custom IC. The input/output voltage is applied to the first comparator terminal and the first pull-up terminal, the reference voltage is applied to the second comparator terminal, and a signal line to be pulled-up is connected to the second pull-up terminal.
In one aspect, a custom integrated circuit (IC) for automatically varying resistance comprises a comparator for comparing a input/output voltage to a reference voltage operatively coupled to a first comparator terminal and second comparator terminal of the custom IC; an NMOS transistor having the output of the comparator operatively coupled to a gate of the NMOS transistor, and a source and a drain of the NMOS transistor operatively coupled across a 2.7 Kxcexa9 resistor; a PMOS transistor having the output of the comparator operatively coupled to a gate of the NMOS transistor and a source and a drain of the PMOS transistor operatively coupled across a 1.0 Kxcexa9 resistor; and the 2.7 Kxcexa9 resistor and the 1.0 Kxcexa9 resistor operatively coupled in series between a first pull-up terminal of the custom IC and a second pull-up terminal of the custom IC. The source of the NMOS transistor and a first side of the 2.7 Kxcexa9 resistor are operatively coupled to the first pull-up terminal of the custom IC, the drain of the NMOS transistor and a second side of the 2.7 Kxcexa9 resistor are operatively coupled to the source of the PMOS transistor and a first side of the 1.0 Kxcexa9 resistor, and the drain of the PMOS transistor and a second side of the 1.0 Kxcexa9 resistor are operatively coupled to the second pull-up terminal of the custom IC. The input/output voltage is applied to the first comparator terminal and the first pull-up terminal, the reference voltage is applied to the second comparator terminal, and a signal line to be pulled-up is connected to the second pull-up terminal. Other aspects and advantages of the invention will be apparent from the following description and the appended claims.